An aircraft typically has one or more potable water tanks on board to accommodate the aircraft's plumbing system. Such water tanks are commonly cylindrical in shape and can range in size depending upon the aircraft and/or the number of tanks on board. In any event, a potable water tank is typically positioned under the cabin floor or other locations on the aircraft which are susceptible to cold temperatures, moisture invasion, and pressure drops/rises caused by changing altitudes.
A heater can be provided to maintain the tank at an acceptable water temperature range and to prevent freezing of the water. In one common type of heater, an electrothermal blanket is shaped and sized to be wrapped around the tank (with openings for plumbing inlets/outlets) and is secured to the tank with appropriately placed lacing hooks. The blanket includes a pattern of wire that forms an electrical resistance heating element connected to a power source on the aircraft to generate the desired heat.
To make the blanket for such a heater, a work platform is provided with pins placed in locations corresponding to the desired heating element pattern. A first layer of a carrier material having appropriately placed pin-accommodating openings is placed on the work platform. The heater wire is then wrapped around the pins to create the desired pattern, and a second layer of carrier material is then placed over the pattern so that the resistance wire is sandwiched therebetween. These and possibly other compiled layers are then cured to encapsulate the resistance wire.
A potable water tank is often made of an electrically conductive material, such as stainless steel or a graphite composition. Accordingly, or in any event, a heating assembly must be designed to guard against electrical shorts. To this end, the carrier layers in the heating blanket are made of an electrically insulating material such as silicone. As long as the carrier layers do not allow the introduction of water or moisture, the heating element circuit will remain electrically insulated.
In the past, heater blankets have incorporated Teflon-coated wire to protect against electrical shorts when a fluid (e.g., hydraulic oil) migrates through the silicone carrier layers. However, the “slickness” of the Teflon coating complicated assembly procedures, particularly the wire-winding process. Specifically, the Teflon-coated wire would not “stick” to a silicon carrier layer (which has a clay-like consistency in an uncured state) during the winding process. To prevent the wire from “jumping” out of the pattern, small tie-down strips of silicone material had to be placed over winding paths throughout the pattern, dramatically slowing the process.
Moreover, the intactness of the Teflon coating was found to be difficult, if not impossible, to obtain during the manufacture of the heating element. Specifically, pins on the work platform would crease or nick the Teflon coating, thereby providing a leakage path. Also, Teflon has a tendency to “cold flow” around pin-imposed corners during the construction of the heating element. Further, damage to the coating can occur from fingernails during handling of the coated wire. Accordingly, even with Teflon-coated wire, the integrity of the carrier layers remains crucial to keeping the heating element electrically insulated.